The present invention relates to power management and, more particularly, managing the peak load drawn by a site owner on an electric utility to reduce the site owner's utility bills.
Utility bills often include a peak load component that assesses a site owner a charge based on the peak usage of grid power at the site during a multi-day billing period.
To reduce the amount of the peak load charge, it is known to deploy a peak load management system that continually measures load at the site throughout the billing period and limits the use of grid power to a target peak load. The target peak load is often statically configured for the billing period, either manually or by an algorithm running on the management system. Whenever the measured load reaches the target peak load during the billing period, the management system discharges stored power from a battery system to keep the use of grid power from exceeding the target peak load. Once the measured load falls below the target peak load, the management system stops discharging stored power from the battery system and the battery system is recharged.
A shortcoming of known peak load management systems is that the target peak load is often configured at a level that is suboptimal. A suboptimal target peak load setting can result from errors in estimating the site owner's power consumption, power generation (e.g., solar output), or both. If the target peak load is set too low, the stored power in the battery system may be exhausted before the measured load falls below the target peak load and the use of grid power may spike above the target peak load, decimating the cost savings from peak load management. On the other hand, if the target peak load is set too high, the use of grid power is permitted to rise above a level that could be successfully offset the battery system, resulting in an underutilization of the battery system that unduly limits the cost savings from peak load management.
One known way to reduce the risk of a spike of grid power use above the target peak load due to battery exhaustion is to build a safety margin into the target peak load. This can be done using various approaches. In one approach, the target peak load is set a predetermined amount above a level where the battery system can successfully offset the predicted peak power use on the “worst case” day of the billing period. An exemplary target peak load with a built-in safety margin is shown in FIG. 1. In the illustrated example, the line at 65 kilowatts, a 10 kilowatt reduction from the expected peak of 75 kilowatts, represents the level where the battery system can successfully offset the predicted peak power use on the “worst case” day of the billing period. The target peak load is set to 67 kilowatts, 2 kilowatts less of a reduction, which provides about a 20% safety margin. In another approach, the predicted peak power use on the “worst case” day of the billing period is padded by a predetermined amount, and the target peak load is set to a level where the battery system can successfully offset the predicted peak power use after padding. In a third approach, the target peak load is set to a level where less than all of the stored power of the battery system is needed to successfully offset the predicted peak power use on the “worst case” day of the billing period.
Unfortunately, building a safety margin into the target peak load does not completely eliminate the risk of a spike above the target peak load due to battery exhaustion. For example, as shown in FIG. 2, if the measured (actual) load substantially exceeds the predicted load on the “worst case” day of the billing period, the stored power in the battery system may be exhausted before the measured load falls below the target peak load, causing the use of grid power to spike above the target peak load despite the built-in safety margin. Such a spike on a single day of the billing cycle can cause a significant increase in the peak load component of the site owner's utility bill and render the peak load management system largely ineffectual. Moreover, building a safety margin into the target peak load has further downside in allowing the peak use of grid power to rise above a level which could potentially be successfully offset by the battery system, which reduces the cost savings achieved by the management system.